Oct demystified

OCT DEMYSTIFIED
TO SEE THE UNSEEN
DR DINESH & DR SONALEE
DRISHTI EYE HOSP INDORE

Optical coherence tomography (OCT)
• OCT is a noninvasive imaging technique that
allows for micrometer resolution examination
of ocular structures & it works similar to
ultrasound, simply using light waves instead
of sound waves.
• By using time-delay information contained in
the reflected light waves , an OCT can
reconstruct a depth-profile of the sample
structure.

OCT Key Features
• High-resolution evaluation of tissue pathology at
the cellular level, achieving axial resolution of up to
2–3 μm in tissue.
• Direct correspondence to the histological
appearance of the retina, cornea, and optic nerve .
• Critical tool in the diagnosis and monitoring of
ocular disease involving the retina, choroid, optic
nerve, and anterior segment .

PHYSICAL PRINCIPLES OF OPTICAL
COHERENCE TOMOGRAPHY
• OCT is based on the Michaelson interferometer
invented in the late 1800s.
• A single beam of white light is split into two beams
moving in perpendicular directions. The beams are
reflected back to, and recombine at, the beam
splitter. When beams recombine, interference
fringes are observed .
• The resulting interference patterns are used to
reconstruct an axial A-scan

PHYSICAL PRINCIPLES OF OPTICAL
COHERENCE TOMOGRAPHY
• Moving the beam of light along the tissue in a line
results in a compilation of A-scans with each A-
scan having a different incidence point.
• From all these A-scans, a two-dimensional cross-
sectional image of the target tissue can be
reconstructed and this is known as a B-scan.
• If these B-scans are repeated at multiple adjacent
positions using a raster scan pattern, then a three-
dimensional volume of structural and flow
information can be compiled.

Principles of OCT Technology
An A-scan is the intensity of reflected light at various
retinal depths at a single retinal location
Combining many A-scans produces a B-scan
A-scan A-scan
+ + . . . =
B-scanA-scans
RetinalDepth
Reflectance Intensity

A-SCAN, B-SCAN, 3D-SCAN
A-scan
B-scan

Types of OCT
• There are two main categories of OCT :
Time-Domain OCT (TDOCT) and Spectral-
Domain OCT (SDOCT). Most early
instruments were based on Time-Domain
OCT technology . Spectral-Domain OCT is
rapidly replacing the Time-Domain
technology in most applications because it
offers significant advantages in sensitivity
and imaging speed.

Spectral-Domain OCT (SDOCT)
• Most of the components are identical to the setup
of the Time-Domain technology. The key difference
is that in an SDOCT system the reference arm
length is fixed.
• Instead of obtaining the depth information of the
sample by scanning the reference arm length, the
output light of the interferometer is analyzed with a
spectrometer (hence the term Spectral-Domain).

• Time domain Optical
Coherence tomography:
• Spectral domain Optical
Coherence tomography:

Time vs Spectral domain OCT
Time domain OCT
• A scan generated sequentially,
one pixel at a time of 1.6
seconds
• Moving reference mirror
• 400 scans/sec
• Resolution – 10 micron
• Slower than eye movement
Spectral domain OCT
• Entire A scan is generated at
once based on Fourier
transformation of
spectrometer analysis
• Stationary reference mirror
• 70,000 scans/sec
• Resolution – 5 micron
• Faster than eye movement
15

HISTOLOGY AND OCT
• Histologically, the retina consists of ten
layers, four of them are cellular and two
are neuronal junctions.
• Most layers can be identified with SD-
OCT . The layers of the retina as seen
on histologic section, in order from the
inner to outer retina, are listed here .

Retinal Layers
• 1 internal limiting membrane (ILM)
• 2 nerve fiber layer (NFL; axons of the ganglion cell layer)
• 3 ganglion cell layer (GCL)
• 4 inner plexiform layer (IPL)
• 5 inner nuclear layer (INL)
• 6 outer plexiform layer (OPL)
• 7 outer nuclear layer (ONL; the nuclei of photoreceptors)
• 8 external limiting membrane (ELM)
• 9 rod and cone inner segments (IS)
• 9 rod and cone outer segments (OS)
• 10 retinal pigment epithelial cells ( RPE )

Four BANDS IN outer retina
• Four bands in the outer retina.
• The innermost band has been attributed to the
external limiting membrane (ELM). This band is
typically thinner and fainter than the others.
• The second of the four bands has been commonly
ascribed to the boundary between the IS/OS
photoreceptors, but a recent consensus that this
band correlates with the inner segment ellipsoid
zone (EZ) .

Four BANDS IN outer retina
• The third band is referred to as either OS tips or as
Verhoeff membrane. This third band correspond to
the contact cylinder between the RPE apical
process and the external portion of the cone outer
segment, and has been called the interdigitation
zone.
• The fourth hyperreflective outer retinal band is
attributed to the RPE, with potential contribution
from Bruch’s membrane and choriocapillaris .

OCT can also produce
a retinal thickness
map.
The OCT software
automatically de-
termines the inner and
outer retinal
boundaries and
produces a false-color
topographic map
showing areas of
increased thickening
in brighter colors and
areas of lesser
thickening in darker
colors

Retinal Thickness
• Different segmentation algorithms from different
instruments tend to follow different borders and
therefore result in different measurements.
• Spectralis SD-OCT instrument follows the posterior
surface of the RPE complex, the Stratus TD-OCT
instrument follows Band #2 ( ellipsoid zone or inner
segment–outer segment (IS/OS) junction ), and the
Cirrus SD-OCT instrument follows the anterior edge
of the RPE layer .

OCTA
• OCT is a noninvasive imaging method
that has been used extensively in the
field of ophthalmology since 2002 .
• OCTA is a functional extension of OCT
and is being used increasingly to detect
microvascular changes in many retinal
diseases since approval by US FDA in
2016.

OCTA
• OCTA is an imaging modality that uses variation (or
decorrelation) in the OCT signal to detect motion in
biological tissues.
• OCTA can noninvasively detect the movement of red
blood cells at capillary-level resolution.
• OCTA is particularly useful for detecting regions of
impaired perfusion and neovascularization.
• OCTA has been used to evaluate many of pathological
macular changes in retinal vascular diseases, including
diabetic retinopathy, retinal vein occlusion, macular
telangiectasia, and neovascular ARMD .

OCTA ADV AND DISADV
• OCTA is at least as good as dye studies for
assessing macular complications of retinal
diseases, such as diabetic retinopathy, retinal
venous occlusion . The main limitation of OCTA is
the field of view, but this is rapidly improving.
• Neovascularisation is detected best by FA .
• Fundus colour photograph is still the gold standard
to grade the severity of diabetic retinopathy .

OCT IN DIFFERENT RETINAL DISEASES
• Differentiate various presentations of
diabetic macular edema
• monitor the course of CSR
• differentiate lamellar / pseudo / full-
thickness macular holes
• Detect macular odema in vascular
occlusions .
• making treatment decisions in ARMD

OCT Findings
in Diabetic Macular Edema
• Kim proposed a classification of five patterns of
DME:
• 1. Diffuse retinal thickening
• 2. Cystoid macular edema
• 3. Serous retinal detachment
• 4. Posterior hyaloidal traction
• 5. Posterior hyaloidal traction with tractional
retinal detachment

1 Diffuse retinal thickening.
• SD-OCT showing
sponge-like swelling,
low reflective,
expanded and irregular
areas of the retina, and
small amount of sub
foveal fluid

2 Cystoid macular edema.
• SD-OCT showing hypo-
reflective fluid-filled
cystic cavities within
outer retinal layers,
separated by hyper
reflective septae of
neuroretinal tissue .

3 Serous retinal detachment.
• SD-OCT showing fluid
accumulation between
the detached retinal
pigment epithelium and
neurosensory retina

4 Posterior hyaloidal traction.
• SD-OCT showing
attached posterior
hyaloid inducing some
tractional effect
possibly exacerbating
the underlying edema.
The hyper reflective foci
with posterior
shadowing represent
small exudates

5 Posterior hyaloidal traction
(more severe form)
• Posterior hyaloidal
traction (more severe
form) with tractional
retinal detachment .

DME
• Of these, the most common pattern is diffuse
retinal thickening (39.5 %), and the least common
are posterior hyaloidal traction (12.7 %) and
tractional retinal detachment (2.9 %) .
• Serous retinal detachment is more common in
males and patients with a high serum triglyceride .
• Patterns that are significantly associated with a
decrease in visual acuity are diffuse retinal
thickening, CME, and posterior hyaloidal traction.

OCT role in DME
•Confirm presence of macular
edema
•Know type of macular edema
•Assess macular thickness
•Vitero macular interface
abnormalities
•Intra retinal exudates

OCT gold standard in monitoring the
progression and treatment response in DME
patients .
Retinal thickness is the most commonly used
quantitative parameter.
CIRRHUS measures the retinal thickness
between ILM & anterior edge of RPE layer .
normal subjects central retinal thickness is
265 µm with CIRRHUS OCT .

Colored Fundus Images vs OCT
• OCT measurements are more sensitive and
reproducible indicator of change in retinal
thickness than color fundus imaging, supporting
the use of OCT as the principal method for
documenting retinal thickness.
• However, OCT is less suitable than fundus imaging
for documenting the location and severity of other
morphologic features of diabetic retinopathy, such
as hard exudates, retinal hemorrhages,
microaneurysms, and vascular abnormalities.

Central Serous Chorioretinopathy
• CSR is an idiopathic syndrome that typically affects
young to middle-aged males and is characterized
by serous detachment of the neurosensory retina.
Focal and multifocal areas of leakage secondary to
increased permeability of the choroidal vessels and
a barrier defect at the level of the RPE have been
described in the pathogenesis of this disorder .

Acute Central Serous
Chorioretinopathy
• OCT shows serous
detachment of the
neurosensory retina
above an optically clear,
fluid-filled cavity,
associated with a
pigment epithelial
detachment.
• Follow up visit at 1 mo
shows decrease in the
amount of subretinal
fluid.

Acute Central Serous
Chorioretinopathy
• Note thickened choroid,
pigment epithelial
detachments & significant
subretinal fluid .
• OCT is also used to
quantify and monitor
amount and extent of
subretinal fluid,
thickening of
neurosensory retina, and
diminution of choroidal
thickening after
treatment

PVD vs VMA vs VMT
• In normal eyes, as the vitreous liquefies due to age, it
detaches from the macula. In some people, an
unusually strong adhesion is present between the
vitreous and macula, and as the vitreous detaches
peripherally, it continues to pull on areas of the
macula.
• The vitreoretinal adhesions transmit tractional forces
to the retina from the vitreous body, having the
potential to cause tensile deformation, foveal
cavitations, cystoid macular edema (CME), limited
macular detachment, or a macular hole. Patients can
present with visual loss and metamorphopsia.

VMA and VMT
• VMA is defined on OCT as “perifoveal vitreous
separation with remaining vitreomacular
attachment and unperturbed foveal morphologic
features.”
• Vitreomacular traction VMT , on the other hand, is
defined by “anomalous posterior vitreous
detachment accompanied by anatomic distortion of
the fovea.” Pseudocysts, cystoid macular edema
and subretinal fluid are typical findings of VMT.

Vitreomacular Traction
PRE SURGERY POST VITRECTOMY

Macular Hole
• Idiopathic macular holes typically occur in the
sixth to seventh decade of life with a 2 : 1 female
preponderance. Symptoms include decreased
visual acuity, metamorphopsia, and central
scotoma. A full-thickness defect in the neural
retina as seen with OCT can differentiate a true
macular hole from a pseudo hole seen clinically.
Pseudo holes are seen in the presence of a dense
sheet of ERM with a central defect that overlies the
foveal center, giving the ophthalmoscopic
appearance of a true macular hole.

GASS Macular Hole STAGES
• Gass stage 1 impending hole is characterized by a
foveal detachment seen as a yellow spot (1A) or ring
(1B) in the fovea . Spontaneous resolution will occur in
approximately 50% of these cases.
• In stages 2–4, there is a full-thickness retinal defect,
with a complete absence of neural retinal tissue
overlying the foveal center.
• What differentiates these stages is the
• Size of the retinal defect (<400 μm in stage 2 and >400 μm in
stage 3)
• or the presence of a complete posterior vitreous detachment
regardless of the hole size (stage 4)

OCT MACULAR HOLE STAGES
• This classification divides macular holes based on the
cause, size of hole, and the presence or absence of
vitreomacular adhesion.
• Full-thickness macular holes can be either primary (if
caused by VMT) or secondary (if caused by other conditions
unrelated to abnormal vitreoretinal traction), and can be
further subclassified by the size of the hole measured on
SD-OCT.
• Based on macular hole width , macular holes are divided as
follows:
• small holes measure 250 μm or less;
• medium size holes are between 250 μm and 400 μm, and
• large holes are larger than 400 μm.

Macular Hole
PRE SURGERY POST SURGERY

Evolution of a macular hole,
visualized with OCT.
• OCT image of a patient
with a peri foveal
posterior vitreous
detachment and no
obvious traction on the
macula.

• B, After 1 year, the
patient experienced
visual distortion; the
image shows obvious
traction with foveal
tractional cavitations.

• C, Image taken 2
months later; note the
full-thickness macular
• hole.

• D, Image taken 1 month
after macular hole
surgery; the hole is
closed. Note the subtle
area of increased
reflectivity in the
center.

• E, Image taken 3
months later shows the
fovea with a nearly
normal contour and
laminar structure.

CRVO
NON ISCHAEMIC
no significant macular edema
ISCHAEMIC cystoid macular
edema with subretinal fluid.

Central retinal vein occlusion
MACULAR ODEMA

Age-Related Macular Degeneration
• AMD is a common cause of irreversible vision loss
among the elderly worldwide.
• AMD can be classified in two forms: non
neovascular (dry) and neovascular (wet or
exudative).
• The non-neovascular form accounts for 80–90% of
cases while the neovascular form accounts for 10–
20% of cases, but was responsible for majority of
severe vision loss (80–90%) prior to widespread
use of VEGF inhibitors.

Age-Related Macular
Degeneration
• OCT may be a useful ancillary test in any stage of
AMD. In patients with dry AMD, the high-definition
averaged B-scans are useful to assess the ultra-
structure of drusen and to examine adjacent retinal
layers that can be compromised by the disease
process.
• The progression of early AMD to severe forms,
such as GA, can be monitored by using OCT.
• The loss of RPE and photoreceptors are easily
observed in the B-scans .

Age-Related Macular
Degeneration
• OCT identifies GA as a bright area resulting from
the increased penetration of light into the choroid
where atrophy has occurred in the macula .
• OCT can be used to identify some of the wet AMD
features, such as the presence of intraretinal or
subretinal fluid, presence of retinal PEDs, which
can be classified in serous , fibrovascular, and
hemorrhagic PEDs.

Early Non-Neovascular AMD:
Drusen
• Drusen appear clinically
as focal white–yellow
excrescences deep to
the retina. They vary in
number, size shape, and
distribution . Drusen are
seen as discrete areas
of RPE elevation with
variable reflectivity .

Retinal pigment epithelium (RPE)
tear

retinal pigmented epithelium
detachment (PED)

CONCLUSION
• Undoubtedly, for many ophthalmologists, not only
for retinal specialists, OCT is the leading tool for
their practice. The number of fluorescein
angiography examinations has been reduced in the
last 10 years with an important increase of OCT
procedures.
• The future will be more interesting with the full
introduction of OCT angiography, wide-field OCT,
and adaptive OCT.

CONCLUSION
• During past two and a half decades, OCT has
evolved to become an essential tool in
ophthalmology. Its ability to noninvasively
image detailed ocular structures and
microvasculature in vivo with high resolution
has revolutionized patient care.
• OCT has changed the approach of
ophthalmologists in their daily practice.

THANK
YOU
DR DINESH
DR SONALEE

Oct demystified

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